The recovery of metals from moving bed cells is known in the art as a very attractive technique, albeit still far from actual industrial practice. Moving bed metal deposition has been first described as an improvement of the more general concept of fluidised bed metal deposition (see for instance U.S. Pat. No. 4,141,804) by Scott et al. in U.S. Pat. No. 4,272,333. A bed of metallic beads is levitated by a liquid electrolyte jet until it passes the top edge of a metal cathode, overflowing in a chamber delimited by such cathode and a semi-permeable diaphragm, separating the falling bed from the anode. The falling bed is thus cathodically polarised, and the metal ions in the electrolyte can discharge on the beads causing their growth. The disclosed method allows to feed the beads as small seeds and to discharge them from the cell after reaching the required growth, but has the obvious drawback of being substantially a batch procedure. Moreover, the cell must be operated as a single cell and has no possibility of being effectively stacked in a laminar arrangement, and its productive capacity by unit volume or by unit installation surface is therefore very limited.
A significant improvement of this concept is offered by the disclosure of U.S. Pat. Nos. 5,635,051 and 5,958,210, directed to the electrowinning of zinc. In this case, the cathodic compartment contains a spouted bed generated by the ascending motion of the electrolyte supplied to a draft tube, and split into two annuli in the falling regions, disposed at the two sides of the tube. The anodic and cathodic compartment are separated by means of an ion-permeable barrier, such as an ion-exchange membrane or the like. The anolyte and the catholyte are therefore physically separated and the growing beads are again excluded from the anodic compartment, but the passage of the ion to be deposited from the anodic to the cathodic compartment is allowed. The cell is somehow better than the one disclosed in U.S. Pat. No. 4,272,333 in terms of productive capacity, being quite flat, and even foreseeing the possibility of a parallel arrangement of a plurality of draft tubes and relevant falling bead annuli to increase the size of at least one dimension thereof. Nevertheless, the deposition disclosed therein is still a typical batch process, the depletion of metal ions in the anolyte chamber having to be counteracted with a delicate restoring procedure, in order to maintain a certain stability of the cell conditions.
A substantial progress with respect to the above described technologies is given by the spouted bed cell disclosed in the co-pending Italian Patent Application MI2002A001524, relative to a cell element which can be laminated in a filter-press structure and which is provided with means for the selection and discharge of the product so as to make possible a continuous-type process, also by means of a peculiar separating element consisting of an electrically insulating diaphragm which operates the exclusion of the beads from the cathodic compartment, while allowing the free passage of electrolyte between one compartment and the other and thus remarkably simplifying the overall balance of matter. In this kind of application, the draft tube which establishes the spouted bed of growing beads is again internal to the cathodic compartment, and the above bed still has an annular-type geometry, with the beads disposed in a generally rectangular annulus for each side of the draft tube (case of the tube centred inside the cathodic compartment) or in a single annulus generally disposed along the single free side (case of the tube arranged along a side-wall). Notwithstanding the good functioning of this type of cell, it however leaves some significant problems unresolved: firstly, the draft tube size is limited by the depth of the cathodic chamber. Since for compactness reasons the latter must have a necessarily reduced thickness (20 mm indicatively), the extension of the bead annuli generated by the spouted bed has a consequently limited planar development. Moreover, the draft tube is sometimes subject to local stoppages or other kinds of functioning irregularities, which are not easy to detect and solve, being the same tube incorporated within the cathode shell. The same can be said for the product selection and discharge system, entrusted to internal devices that are difficult to control in case of even partial stoppages. Finally, to maintain a spouted bed with the required characteristics, the draft tube inlet becomes a zone of very high turbulence where the friction phenomena, locally reverberating on the diaphragm, entail remarkable hazards of damaging or rupture.